The present invention relates to a battery separator. More particularly, the present invention relates to a battery separator which is capable of preventing a short circuit in batteries to reduce the poor quality rate of the batteries.
A separator has been used between an anode and a cathode in alkaline batteries to separate them for preventing short-circuiting and also to hold an electrolyte for carrying out a smooth electromotive reaction.
Recently, a space allotted for the battery has been becoming smaller in electronic instruments due to the need for miniaturization and weight-saving. Nevertheless, the performance and property requirement for such a battery is the same as or even more higher than that for a conventional battery, and therefore, there has been a demand to increase the capacity of the batteries. For such a purpose, it is necessary to increase the amount of active materials in electrodes whereby a volume occupied by the separator therein is to inevitably be reduced. In other words, a thickness of the separator is to be made thinner. However, if the conventional separator is simply made thin, there are problems that the separator may be liable to be broken by tension during a step of assembling plates for the production of batteries (composed of plate groups), a plate flash or flashes may go through the separator or the separator may be liable to be torn off by a plate edge or edges, said problems causing a short circuit whereby the yield of batteries will become poor.
In order to solve such problems, for example, JP, 10-284042, A (1996) discloses a separator using a fiber with a high strength in which the strength of a single fiber is 5 g/d (denier) or more. Although such a prior art separator is capable of improving situations as aforementioned, there has been still a demand for a separator where the above-mentioned problems are far less caused.
The present invention has been carried out for solving the above problems. An object of the present invention is to provide a battery separator which will hardly be broken by tension during the manufacturing stage of batteries (composed of plate groups), will hardly be punctured with a plate flash or will hardly be torn off by a plate edge so that a short circuit will hardly take place whereby batteries will now be able to be manufactured in a stable manner.
A battery separator (also hereinafter briefly referred to as xe2x80x9cseparatorxe2x80x9d) according to the present invention is equipped with a fiber sheet containing polypropylene fibers with a tensile strength of not less than 10 g/d (denier) and a Young""s modulus of not less than 600 kg/mm2 (hereinafter, referred to as xe2x80x9chigh Young""s modulus polypropylene fibersxe2x80x9d), said separator having a resistance to penetration by edge with a force of not less than 585 gf (i.e., its puncture strength: not less than 585 gf). The inventors have found that the separator equipped with a fiber sheet containing the high Young""s modulus polypropylene fibers, said separator having its puncture strength of not less than 585 gf, hardly causes accidents including the breakage of the separator by tension applied during the manufacturing stage of batteries (composed of plural plates), the perforation of the separator with a plate flash or flashes, and the tearing of the separator by a plate edge or edges, leading to the occurrence of a short circuit, whereby the batteries can now be manufactured in a stable manner.
Another separator of the present invention is equipped with a fiber sheet containing not less than 10 mass % of polypropylene fibers (high Young""s modulus polypropylene fibers) with a tensile strength of not less than 10 g/d (denier) and a Young""s modulus of not less than 800 kg/mm2. The inventors have found that, since the said high Young""s modulus polypropylene fiber has excellent strength and elasticity, the separator equipped with a fiber sheet containing not less than 10 mass % of the said high Young""s modulus polypropylene fiber hardly causes problems that the separator may be broken by tension during the manufacturing stage of batteries (composed of plural plates), the plate flash may go through the separator and the separator may be torn off by the plate edge, said problems leading to a short circuit, whereby batteries can now be manufactured in a stable manner.
In the case of fiber sheets each containing not less than 20 mass % of fusible fibers, separator""s tensile strength and bending resistance are improved whereby problems hardly occur that the separator may be broken by tension during the manufacturing stage of the batteries (composed of plate groups), the plate flash goes through the separator and the separator is torn off by the plate edge, said problems resulting in a short circuit.
Especially when the tensile strength of the fusible fiber is not less than 3 g/d (denier), events more hardly occur that the separator may be broken by tension during the manufacturing stage of the batteries (composed of plate groups), the plate flash goes through the separator and the separator is torn off by the plate edge, said events resulting in a short circuit.
In addition, when the high Young""s modulus polypropylene fiber is thicker than the fusible fiber, the said high Young""s modulus polypropylene fibers form a skeleton in a state of being bonded via fusing with the fusible fibers. Therefore, even if a deformation happens, it can be retained within a deformation of the fused fiber. Accordingly, the thickness of the separator in the batteries can be maintained, spaces can be secured and the good permeability of gas is achievable whereby the inner pressure of the battery can be reduced when it is used as a separator for closed type batteries. Further, since the thickness of the separator can be maintained, the electrolyte is hardly squeezed out and the deficiency of electrolyte can be suppressed whereby there is another effect that the life of the batteries can be extended.
Further, when the low-melting component in the fusible fiber in high-density polyethylene, a strong fusion is achieved and an excellent tensile strength is obtained as well. Therefore, events more hardly occur that the separator may be broken by tension during the manufacturing stage of the batteries (composed of plate groups), the plate flash goes through the separator, and the separator is torn off by the plate edge, said events leading to a short circuit.
Furthermore, in the case of the fiber sheet containing not less than 10 mass %, and more preferably not loss than 20 mass %, of superfine polyolefin fibers, a property of retaining the electrolyte can be improved and a deficiency of the electrolyte can be suppressed whereby there is an effect that the life of the batteries can be extended.
The above-mentioned polypropylene fiber (high Young""s modulus polypropylene fiber) is preferably a species having a thermal shrinkage ratio of 8% or less at the temperature of 140xc2x0 C. When the thermal shrinkage ratio is 8% or less, the resulting separator has an excellent uniform fiber dispersibility due to reduced changes in the size of the separator even when heat is applied during the manufacture of the separator. Therefore, the separator has quite better characteristics in terms of the aforementioned properties.
It is preferable that the cross sectional fibrous shape of the above-mentioned polypropylene fiber (high Young""s modulus polypropylene fiber) is non-circular. When the cross sectional fibrous shape is not circular, the above-mentioned properties become better. This is believed to be explained by the presumption that, even when the plate f lash or edge touches the high Young""s modulus polypropylene fiber, the high Young""s modulus polypropylene fiber hardly slips and a dislocation at a contacting point of the fiber is suppressed whereby a force from the flash and the edge may be received in a dispersed manner. In addition, when the high Young""s modulus polypropylene fiber is non-circular in cross section, the fiber sheet can be in a densified structure, thereby facilitating the preparation of a thinner separator.
It is also preferable that the above-mentioned polypropylene fiber (high Young""s modulus polypropylene fiber) is fibrillatable. If fibrillatable, it is far better in view of the above-mentioned properties, this is believed to be explained by the presumption that, even when the plate flash or edge touches this high Young""s modulus polypropylene fiber, the said high Young""s modulus polypropylene fiber is fibrillated and can catch the force from the flash or the edge whereby the penetrating force by the edge and flash hardly acts.
When the battery separator possesses the resistance to puncture by needle with a force of 700 gf or more, the plate flash more hardly penetrates the separator showing an excellent resistance to short circuiting.
When a texture index of the battery separator is 0.2 or less, the fiber (particularly, high Young""s modulus polypropylene fiber) is in a uniformly dispersed state, whereby events hardly take place that the separator is puncture with the plate flash and is broken by the plate edge, said events causing a short circuit.
When the thickness retention ratio of the battery separator is 86% or more, it is possible to retain the thickness of the separator in the battery. A void can also be secured therein and the good permeability of gas can be achieved. Therefore, when it is used as a separator for closed-type batteries, the inner pressure of the battery can be reduced. In addition, since the thickness of the separator can be retained, the electrolyte is hardly squeezed out and the deficiency of the electrolyte can be suppressed whereby there is another effect that the life of the batteries can be extended.